Although a variety of electrical and electronic sensors are known for determining the position of a mechanical element, such devices generally suffer from susceptibility to natural and man-made electromagnetic noise and other environmental effects that can degrade their performance. For this reason, electrically passive optical position sensors offer a clear advantage for use in extreme environments and in applications where very high reliability is important. For example, optical sensors will soon be employed on aircraft for sensing the position of control surfaces and may be incorporated into a servo system in which the position command and the position feedback information are generated by two similar optical sensors.
Either digital or analog encoding techniques can be used in an optical position sensing system to precisely determine the position or monitor movement of a ratary shaft or linear actuator. In these systems, light signals are usually conveyed to and from the sensors by optical fibers. Typically, light propagating through an optical fiber from a remote source is modulated by an encoded track on a mechanical element that rotates or moves linearly. The modulated light signal is conveyed to a light sensor that determines the position of the mechanical element based on the modulated intensity of the light signal.
Both the reflective and transmissive properties of an encoded track have been used in prior art devices for analog modulation of a light signal to sense position. In the case of transmissive modulation, the density of the encoded track varies as a function of its position relative to the light beam passing through it. Alternatively, the reflectivity of the encoded track can be varied with the position of the element relative to the incident light beam so that the intensity of light reflected from the encoded surface determines the surface position. In either case, the optical fiber that conveys the modulated light signal from the light sensor is disposed so that the light modulated by the encoded track is directed into it.
Since the intensity of light reaching the remote light sensor is determinative of the position of the mechanical element, any variation in light intensity not caused by the reflectivity or transmissivity of the encoded track represents an error in this determination. For example, instability in the light source intensity or in the sensitivity of the light sensor, variable light losses in the optical fiber interconnections, or contamination of the exposed optical surfaces of the position sensor can introduce a variation in the light intensity perceived at the light sensor, and can thus contribute a significant error in the position determined by the sensor. Furthermore, any such error occurring after the position sensor is calibrated is not readily detectable.
An analog optical position-indicating sensor is disclosed in U.S. Pat. No. 4,769,536 that attempts to compensate for this type of error. In this sensor, light at three different wavelengths is conveyed through a common optical fiber and directed through an encoded track disposed on a movable element. The encoded track is completely transparent to light at two of the wavelengths, but its transmissivity with respect to the third wavelength varies with the position of the encoded track. Light transmitted through the encoded track is conveyed through another optical fiber to two optical couplers disposed adjacent to three light sensors. The optical couplers divide the light into three separate beams, each comprising light at one of the three wavelengths, and direct these beams to the light sensors, which determine the relative intensities of the light at each wavelength. By monitoring the ratio of the various light beam intensities at the light sensors, modulation of light intensity at the third wavelength by the encoded track, and thus, the position of the element can be determined independently of any spurious variations in light intensity that occur in the system. However, since wavelength discrimination occurs at the encoded track, any variation in its transparency with respect to the two wavelengths that it is not intended to modulate (e.g., due to contamination by dirt) causes an error in the position measured by this device. Because the area of the encoded track is typically relatively small, it is more susceptible to the effects of contamination than a larger encoded area would be. A further disadvantage of this optical position sensor is its relative complexity.
In a commonly assigned U.S. Pat. application Ser. No. 574,203, filed on Aug. 28, 1990, now U.S. Pat. No. 5,068,528, an encoded surface position sensor is disclosed that compensates for variations in the intensity of light signals propagating through the sensor system due, for example, to contamination of optical surfaces in the sensor or variations in the source. This position sensor uses light at a reference wavelength and at a test wavelength that propagate along a common optical path until separated by an interference filter. A test beam comprising light at the test wavelength is transmitted through the interference filter toward an encoded surface. The encoded surface reflects a portion and transmits another portion of the test beam. The transmitted portion of the test beam is then reflected by a mirror, which is disposed adjacent to a surface of a rotatable disk or a linear encoder opposite from that on which the encoded surface is applied. The mirror reflects the transmitted portion of the test beam along a first optical path, while the portion of the test beam that is reflected by the encoded surface travels along a second optical path. The relative transmissivity/reflectivity of the encoded surface varies with its position in respect to the point on the surface at which the test beam is incident, thereby varying the intensity of the reflected and transmitted portions of the test beam in a predefined manner. A reference beam comprising light at the reference wavelength is reflected by the interference filter along a reference path and is then split by a beam splitter. A portion of the reference beam is transmitted toward a mirror, which reflects it back toward the interference filter. The interference filter reflects this portion of the reference beam along the first optical path. The portion of the reference beam reflected by the beam splitter is also reflected by the interference filter so that it travels along the second optical path with the portion of the test beam reflected by the encoded surface. Light traveling along the first and the second optical paths is conveyed by optical fibers to a sensor assembly that determines the relative intensities of the transmitted and the reflected portions of the test and reference beams to define the position of the encoded surface, thereby compensating for light losses in the optical fibers and other parts of the system.
One of the problems with the optical position sensor just described is its sensitivity to differences in the optical losses occurring in the test and reference light paths. Since both the reference and test light beams are split apart and then recombined, the elements of the optical system that define their paths must be precisely aligned. Although optical trimming elements can be employed to correct minor alignment problems that arise during the manufacture of the sensor, use of such corrective measures complicates the assembly process and increases its cost.
It is also desirable to incorporate some form of self-monitoring to determine if an optical sensor is operating properly. This type of fault detection can be accomplished, for example, by monitoring the sum of the transmitted and reflected light from the sensor to determine if the sum of these two components remains substantially constant. In a reflective-only encoder configuration, this self-monitoring function can be implemented by providing two encoded tracks with complementary characteristics. A further object of at least one embodiment of this invention is to provide a determination of whether the optical sensor is functioning properly, where such determination is generally independent of slight misalignment of the paths that are followed by the test and reference beams. These and other objects and advantages of the invention will be apparent from the attached drawings and the Detailed Description of the Preferred Embodiment that follows.